Microfluidic pumping and dispensing of liquid chemical reagents is the subject of three U.S. Pat. Nos. 5,585,069; 5,593,838; and 5,603,351, all assigned to the David Sarnoff Research Center, Inc. The system uses an array of reservoirs, with connecting microchannels and reaction cells etched into a substrate. Electrokinetic pumps comprising electrically activated electrodes within the capillary microchannels provide the propulsive forces to move the liquid reagents within the system. The electrokinetic pump, which is also known as an electroosmotic pump, has been disclosed by Dasgupta et al., see "Electroosmosis: A Reliable Fluid Propulsion System for Flow Injection Analyses", Anal. Chem. 66, pp. 1792-1798 (1994). The chemical reagent solutions are pumped from a reservoir, mixed in controlled amounts, and then pumped into a bottom array of reaction cells. The array may be decoupled from the assembly and removed for incubation or analysis.
The above described microfluidic pumping can be used as a printing device. The fluids pumped become ink solutions comprising colorants such as dyes or pigments. The array of reaction cells may be considered ink delivery chambers to be used for picture elements, or pixels, in a display, comprising mixtures of pigments having the hue of the pixel in the original scene. When contacted with paper, the capillary force of the paper fibers draws the dye from the cells and holds it in the paper, thus producing a paper print, similar to a photograph, of the original scene.
The inks that can be used in a microfluidic printing apparatus as described above have been disclosed in the above referenced U.S. Patent Applications. These inks can be dispersions of colorants in common solvents. Examples of such inks may be found is U.S. Patent No. 5,611,847 by Gustina, Santilli, and Bugner. Inks may also be found in the following commonly assigned U.S. patent application Ser. Nos. 08/699,955, 08/699,962, and 08/699,963, all filed Aug. 20, 1996 by McInerney, Oldfield, Bugner, Bermel, and Santilli; 08/790,131, filed Jan. 29, 1997 by Bishop, Simons, and Brick; and 08/764,379, filed Dec. 13, 1996 by Martin. In a preferred embodiment of the invention the solvent is water. Colorants such as the Ciba Geigy Unisperse Rubine 4BA-PA, Unisperse Yellow RT-PA, and Unisperse Blue GT-PA are also preferred embodiments of the invention. The cross referenced application "Stable Inks for Microfluidic Printing" also discloses inks made by solution of dyes in a solvent
A difficulty associated with the above described microfluidic printing is the control of ink transfer from the microfluidic printing apparatus to the receiver. Since the inks are in fluid form at room temperature, the flow of the inks to the receiver needs to be terminated in an accurate fashion so that the correct amount of inks are transferred to the receiver. Otherwise, an excessive amount of ink can be drawn from the ink delivery chambers to the receiver, which tends to produce image defects such as ink coalescence and color bleeding. The ink flow control can be done by mechanically separating the receiver from the microfluidic printing apparatus, which is difficult to control because the flow-rate can vary as a function of temperature. Another approach to control ink flow is to install micromechanical devices such as microvalves, which is disclosed in U.S. patent application Ser. No. 08/868,102, filed Jun. 3, 1997. While this method is capable of solving the above described problem, it is desirable to control ink transfer without the additional complexity of the microvalves.